To me its not even double standards because what's happening between these sets of characters is not the same. Gin is developing feelings for her which is mixing with his spark of inspiration with Sakura as his muse but she really isn't doing anything to show that she's romantically interested in him at all. She just has an unhealthy mindset of how to motivate Gin to get better at drawing because that's what she thinks is going to get him to care enough to practice. It may or may not still be manipulation, but we don't know if she understands how to actually be a good teacher, and I think having that single panel of Sakura's teacher means that they'll get into it at some point in the future, it seems significant enough that we got to see what he looks like.
The glasses teacher Okuda and Maki is just straight up an inappropriate student-teacher relationship and there was literally a single page depicting it so far, I think the mangaka has the capacity to write something more nuanced than what you're giving her credit for, at least wait for another chapter. I don't think we're supposed to see these relationships as the same thing.
can't believe ya'll made me make register my account so i could comment on the forums LOL, I think this series is great so far just let her cook!!!